Composite fabrics, dry or pre-preg (pre-impregnated resin systems), including fiberglass, carbon fiber, graphite, ceramics, Kevlar, Aramid and other hybrids of uni-directional or bi-directional makeup have been used in the construction of a variety of products, including aircraft components, marine applications, automotive and various industrial and appliance applications to reduce weight while improving structural properties and aesthetic flexibility. These products lend themselves to products in size and shape typically not less than one square foot in surface area and that may be flat, faceted, concave, convex or a combination thereof.
Typically in applications where these products are used, the lay-up process is performed by hand. Exceptions include chopper gun technology, tape-laying and fiber placement machines. To achieve the desired structural properties with the use of chopper gun technology the application of more product in a given area is the most common method; however, disadvantages of this method include increased weight of the final product and limited ability of the result to be an engineered product. Further, robots have been employed to expedite and provide for a more-uniform and repeatable application. In either method of application this process tends to be very messy.
Tape laying machines have proven to be very accurate and reliable; however, because of the size and extreme costs associated with them, only a limited number of companies can provide product via this method. Plugs, molds and mandrels employed within the ‘work cell’ or envelope of the system must be very precisely built and placed. Set-up can be very costly and time consuming due, in part, to the method by which the systems are programmed.
These systems are also limited, in that the plug, mold or mandrel must typically be placed ‘in position’ or that the tape is always placed with gravity assist. Exceptions to this may be where the ‘tack and drape’ of the product being used lends itself otherwise.
A limiting factor with these machines is that the tape dispensing rolls have been limited in size, allowing for only 1.00″ to 12.00″ of material to be laid in a single pass, and subject to the amount of engineered overlap per seam. Because of this limitation, additional weight is added to the product being constructed. This, in turn adds to the time required for the lay-up itself and becomes an issue with respects to the out time allotted for a given resin system.
These machines do not lend themselves to the lay-up of dry fabrics such as those used in the construction of boat or ship hulls and deck components. Autoclaves large enough to house such products are impractical and therefore limit the amount of pre-preg material that can be used, even by hand, unless mobile IR (infrared) modules are placed about the product for curing purposes. These products are generally large and multi-faceted and/or concave, convex or combination surfaced and do not lend themselves to the work envelopes of commercially available tape-laying machines of today. As a result, the majority of such lay-up work is performed by hand and vacuum bagging is performed locally.
In many instances lay-up schedules established by design and engineering requirements for large parts are performed by hand with the mechanics working in concert with overhead lasers to position fabric edges in a given direction. To facilitate this process, fabric profiling and cutting machines have flourished in concert with the lay-up regime. These X-Y gantry systems have come of age providing various cutting systems including slitting, flying knives, waterjet, laser and ultrasonic tools to perform the required cuts and profiles for a given lay-up.
Again, the issue of out time for a given matrix and resin system requires diligence by the mechanic whose individual reach may limit his or her ability to place the fabric in the correct position without getting up and onto the plug or mold. Scaffolding, mobile platforms or man-lifts would then be utilized to facilitate the lay-up regime.
Typically, once the fabric for a specific lay-up has been cut or profiled, it must be returned to cold storage, typically 40 degrees Fahrenheit or cooler, until it can be laid up. This is due, in part, to the amount of time that has been used to roll out, cut or profile and label by schedule, and the additional issue that the plug or mold may be in use and not available to perform the lay-up. Even with automated versions of the cutting and profiling machines, the rate at which the human roll is performed limits the ‘thru-put’ of the entire process. This process is further complicated when different types of fabrics are selected by design for a given lay-up schedule.
As a result of this current lay-up process, regardless of the component being constructed, thru-put is greatly reduced because of multiple handling of the material, especially where ISO standards are practiced. These limitations therefore tend to drive up the part costs per square foot of lay-up exponentially due to component size.
With fiber placement machines specific fiber matrices and resin systems are typically placed in many applications employing mandrels, whether vertically or horizontally positioned. Cylindrical and conical shaped products lend themselves well to this method. Fiber placement machines have also been employed in the process of making up flexible products, such as sails used in marine applications that are typically complex in shape and surface geometry. In these applications mobile IR panels perform the cure process upon completion of the fiber placement. Again, such machines are limited in the amount of material that can be laid up in a given pass or path, coupled with the typically slow placement speeds.
These machines require a great deal of attention and monitoring as there are issues relating to broken fibers, resin viscosity and impregnation, not to mention the programming flexibility limitations imposed by currently used motion control software systems.
Upon examination of the aforementioned composite application, lay-up and/or placement systems currently being utilized by the various industries discussed, four areas of concern continue to plague the practical implementation and use of advanced composite construction systems, namely, human labor, out-time limitations of the material itself, flexibility of the process or system for lay-up and the enormous costs associated with the equipment currently available.